1. Field of the Invention
The present invention relates to a thermal flow rate sensor, and more particularly to a thermal flow rate sensor supplying a digital output.
2. Description of the Background Art
A thermal flow rate sensor detecting a flow rate of a fluid by electrically detecting an amount of heat transmitted to the fluid has been known.
For example, Japanese Patent Laying-Open No. 2002-005717 (Patent Document 1) discloses a thermal flow rate sensor, including a first temperature detection resistor formed on a substrate on a side upstream with respect to a direction of flow of a fluid, a second temperature detection resistor formed on the substrate on a side downstream with respect to the direction of flow of the fluid, a heat-generating resistor formed on the substrate to heat the first and second temperature detection resistors, and a first fluid temperature detection resistor and a second fluid temperature detection resistor formed on the substrate at positions not affected by the heat-generating resistor, in which each resistor and one fixed-value resistor or a plurality of fixed-value resistors are connected to each other to form a bridge circuit, and a flow rate of the fluid is determined by detecting a heating current that is correlated with the flow rate of the fluid and is fed through the heat-generating resistor such that the bridge circuit returns to a balanced state from an unbalanced state that has been caused by variation in the temperature of the first temperature detection resistor and the second temperature detection resistor in accordance with the flow rate of the fluid. According to such a configuration, a temperature characteristic of a heat dissipation coefficient having flow rate dependency can effectively be compensated for and temperature dependency of a sensor output can be suppressed.
On the other hand, as the thermal flow rate sensor according to Patent Document 1 supplies an analog output, an AD (Analog to Digital) converter should be provided in order to obtain a digital output for connection to a computer, a CPU (Central Processing Unit) and the like, and a circuit configuration has been complicated due to mixed presence of an analog circuit and a digital circuit.
It is noted that the digital output hereinafter refers to a signal having “1” or “0” that can be taken in the computer, the CPU and the like and processed therein.
In order to solve the above-described problems, for example, Japanese Patent Laying-Open No. 03-042534 (Patent Document 2), Y. Pan et al., “A New Class of Integrated Thermal Oscillators with Duty-cycle Output for Application in Thermal Sensors,” Sensors and Actuators A21-A23 (1990) pp. 655-659 (Non-Patent Document 1), and H. Verhoeven et al., “Design of integrated thermal flow sensors using thermal sigma-delta modulation,” Sensors and Actuators A52 (1996) pp. 198-202 (Non-Patent Document 2) disclose a thermal flow rate sensor adopting ΣΔ control method as the thermal flow rate sensor capable of obtaining a digital output.
It is generally demanded that the digital output of the sensor attains high resolution and quick response. The thermal flow rate sensors according to Patent Document 2, Non-Patent Document 1 and Non-Patent Document 2, however, do not include a configuration for obtaining a digital output attaining high resolution and quick response as the sensor output. For example, in the thermal flow rate sensor according to Non-Patent Document 2, a 10-bit sensor output is updated every 20 msec as shown in Table 1 of Non-Patent Document 2. Namely, the output of the thermal flow rate sensor according to Non-Patent Document 2 merely has resolution of 10 bits and frequency response of 50 Hz.